Chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) are tumors of mature B cells and are closely related biologically and in clinical behavior. Both are currently incurable with chemotherapy. Because many patients are older, application of allogeneic transplantation and tolerability of aggressive chemotherapy is limited. Median survival for patients with CLL is 10 years and for MCL patients ranges between 5 and 6 years. Thus, there is a need to develop novel treatments, especially targeted agents and more tolerable immunotherapeutic approaches. The recognition that the B-cell receptor (BCR) repertoire expressed on CLL cells is highly skewed led to the hypothesis that antigen selection plays a role in disease pathogenesis. Frequently, these antigens appear to be auto-antigens leading to the concept that CLL is a malignancy of auto-reactive B-cells. Recent data suggest that the BCR repertoire in MCL is also skewed and by extension, that MCL similar to CLL is a disease of antigen-selected B cells. BCR signaling has emerged as the pivotal pathway in the pathogenesis of CLL. A major contribution from my group has been the first demonstration of active BCR signaling in CLL patients in vivo. Our findings support the importance of the BCR for disease progression and identify the pathway as a relevant target for therapeutic intervention. Furthermore, we showed that BCR signaling and the consequent activation of the NF-B pathway occurs primarily in the lymph node microenvironment rather than in the peripheral blood or bone marrow. Thus, in key aspects, the biology of CLL is shaped by its environment; an insight that changes our therapeutic approach, the design of correlative studies, and the development of model systems. We have shown that BCR signaling and activation of the NF-B pathway in CLL cells occurs primarily in the lymph node microenvironment. To expand on these observations, we conducted a clinical study using deuteriated water to label the proliferative fraction of the CLL clone in vivo (NCT01117142), which provided direct in vivo evidence for increased tumor proliferation in the lymph node. Furthermore, in collaboration with Dr. Chiorrazzis group we could show that a distinct subpopulation of CLL cells in the lymph node contains the proliferative core of the disease. Several lines of evidence indicate that BCR signaling and the microenvironment are equally important in the pathogenesis of MCL as they are for CLL. The impressive clinical activity of inhibitors of BCR signaling in MCL further suggests an important role of this pathway in MCL. However, direct evidence for activation of BCR signaling in MCL has been missing. Over the past year we completed an analysis of primary tumor samples from blood and lymph nodes of MCL patients. Using gene expression profiling and flow cytometry we could show that MCL cells in the lymph node signal through the BCR leading to activation of downstream pathways including the NF-&#954;B pathway. These data provide evidence for a dependence of MCL on BCR signaling and provide the basis for targeted therapy with inhibitors of BCR signaling in MCL. We are currently evaluating whether tumor specific mutations in BCR and / or NF-&#954;B signaling contribute to activation of these pathways. Having shown the importance of the lymph node microenvironment for CLL cells, we developed a preclinical model system that can reproduce crosstalk between tumor and host microenvironment. As there are currently no good cell line or mouse models of CLL, we adapted a recently described mouse xenograft model and validated that human CLL cells engrafting in the murine spleen proliferate and undergo activation of BCR and NF-B pathways, similar to what we have previously found in the human lymph node. The requirement of the tissue microenvironment for full activation of the BCR on CLL cells, as we have demonstrated in the human lymph node, suggests a role for additional cell types or co-stimulatory molecules in vivo. We are using the NSG CLL xenograft model to investigate whether specific antigens or host factors or the addition or elimination of distinct human cell populations such as T cells will modulate the survival and proliferation of xenografted CLL cells. Several small molecule inhibitors of BCR signaling and of PI3K have entered clinical development in CLL and other hematologic malignancies. Currently, the most promising early clinical results have been achieved with fostamatinib (an inhibitor of SYK), ibrutinib (an irreversible inhibitor of BTK) and idelalisib (an inhibitor of PI3K). From January 2012 to January 2014 we accrued 86 patients into our single agent ibrutinib trial (NCT01500733). We recruited patients with an unmet clinical need, either elderly patients who are often unable to tolerate current aggressive standard therapies, or CLL with a deletion of chromosome band 17p13.1 (del 17p). The latter patients have a particularly poor outcome with chemotherapy and are in greatest need for novel approaches. We found that Ibrutinib induced responses in over 90% of patients irrespective of del 17p that were durable. At a median follow-up of 24 months none of the patients without del 17p has progressed. The estimated rate of progression for previously untreated patients with del 17p who received Ibrutinib as first-line therapy was 9%. The estimated rate of progression for relapsed or refractory patients with del 17p who received Ibrutinib as salvage therapy was 20%. These results are much better than what can be achieved with current standard of care chemotherapy regimens and suggest that Ibrutinib may become the preferred agent for these patients. We completed a detailed biologic analysis on the impact of Ibrutinib on the tumor cells in the blood and lymph nodes of patients with CLL treated on our clinical trial. We validated the on-target effect by demonstrating inhibition of BCR and NF-&#954;B signaling and decreased tumor proliferation. Interestingly, the degree of inhibition of BCR signaling in the lymph node positively correlated with the degree of reduction in lymphadenopathy. Because loss of function mutations in BTK causes a severe immune defect known as Brutons agammaglobulinemia, assessing the impact BTK inhibition on immune function is important. We found that immunoglobulin (Ig) G levels remained unchanged on treatment, while IgM and IgA increased. This may help prevent or successfully control infections and may explain why the frequency of infections decreases with increasing duration on treatment. We are now in the processing of characterizing shifts in normal B cells and T cells during treatment that may lead to insights how a recovering normal immune system may be harnessed to improve disease control.